The contrasting role of auxin in submergence-induced petiole elongation in two species from frequently flooded wetlands

The involvement of auxin in the submergence-induced petiole elongation has been investigated in Rumex palustris and Ranunculus sceleratus. Both wetland species are capable of enhanced petiole elongation upon submergence or treatment with exogenous ethylene (5μl l⁻¹). Treatment of intact Rumex palustris plants with 1-naphthalene acetic acid (NAA) at 10⁻⁴ M enhanced petiole elongation, while treatment with N -1-naphthylphthalamic acid (NPA) had no effect on petiole elongation. The elongation response after NAA or NPA treatment was comparable for plants in both submerged and drained conditions. Pre-ageing of detached petioles of Rumex palustris for 3 h in light or in dark conditions had no effect on the submergence-induced elongation. In comparison to intact plants, detached petioles of Rumex palustris , with or without lamina, did not show significant differences in responsiveness to IAA between drained or submerged conditions. This was in contrast to Ranunculus sceleratus where submergence caused a clear increase in responsiveness towards IAA. Removal of the lamina, the putative source of auxin, or treatment with NPA did not hinder the submergence-induced elongation of detached Rumex palustris petioles, but severely inhibited elongation of detached Ranunculus sceleratus petioles. This inhibition could be restored by application of NAA, suggesting the specific involvement of auxin in the submergence response of Ranunculus sceleratus. It is concluded that, in contrast to Ranunculus sceleratus , auxin is probably not involved in the submergence-induced petiole elongation of Rumex palustris.     

Early embryo development in Fucus distichus is auxin sensitive

Auxin and polar auxin transport have been implicated in controlling embryo development in land plants. The goal of these studies was to determine if auxin and auxin transport are also important during the earliest stages of development in embryos of the brown alga Fucus distichus. Indole-3-acetic acid (IAA) was identified in F. distichus embryos and mature tissues by gas chromatography-mass spectroscopy. F. distichus embryos accumulate [(3)H]IAA and an inhibitor of IAA efflux, naphthylphthalamic acid (NPA), elevates IAA accumulation, suggesting the presence of an auxin efflux protein complex similar to that found in land plants. F. distichus embryos normally develop with a single unbranched rhizoid, but growth on IAA leads to formation of multiple rhizoids and growth on NPA leads to formation of embryos with branched rhizoids, at concentrations that are active in auxin accumulation assays. The effects of IAA and NPA are complete before 6 h after fertilization (AF), which is before rhizoid germination and cell division. The maximal effects of IAA and NPA are between 3.5 and 5 h AF and 4 and 5.5 h AF, respectively. Although, the location of the planes of cell division was significantly altered in NPA- and IAA-treated embryos, these abnormal divisions occurred after abnormal rhizoid initiation and branching was observed. The results of this study suggest that auxin acts in the formation of apical basal patterns in F. distichus embryo development.     

Is Cell Elongation Regulated by Extracellular Auxin?

Experiments with isolated epidermal strips of maize coleoptiles, pretreated with auxin and further incubated on sucrose agar containing different concentrations of auxin (indole-3-acetic acid, IAA or naphthalene-1-acetic acid, NAA) and/or naphthylphthalamic acid (NPA), are described. Preincubation for 2h with 2 . 10^?4M IAA or 10^?5M NAA in buffer, followed by 30 min wash in buffer results in measurable cell elongation during a subsequent incubation for 6 h on sucrose agar. Addition of 10^?4M NPA inhibited the response to auxin and this inhibition could be reversed by providing IAA in addition to NPA. Inner tissue fragments (without outer epidermis) did not respond to external IAA. These results lead to the conclusion that auxin secretion at the outer epidermis may be an essential step in auxin-regulated coleoptile growth.     

Ascorbic acid as negative effector of the peroxidase-catalyzed degradation of indole-3-acetic acid

Ascorbic acid is a strong inhibitor of indole-3-acetic oxidation catalyzed by commercial horse-radish peroxidase. In the presence of excess ascorbic acid, the indole-acetic acid oxidation catalysis is apparently blocked. The activity of peroxidase for indoleacetic acid at pH 3.7 and 33°C, in the presence of 2,4-dichlorophenol and MnCl₂ as promotors was measured by polarographic technique. The K_m was 0.27 m M and the maximum velocity was 1.02 mmol O₂ (mg protein)⁻¹ min⁻¹. Dixon plots lead to an apparent K_i of 1.25 (μ M for ascorbic acid and the inhibition was apparently competitive. Ascorbic acid, besides appearing to be a strong inhibitor of the IAA oxidase activity of peroxidase, seemed to protect IAA from total degradation. Addition of more than 5 μ M ascorbic acid produced both an exponential increase in the lag time before the onset of reaction and, at the end, an oxidation protection of 26 μ M IAA when 111 μ M IAA was present at the stawrt. The possibility of ascorbic acid-IAA auxin from endogenous oxidation in plants, is proposed.     

Variation in indole-3-acetic acid transport and its relationship with growth in etiolated lupin hypocotyls

The relationship between the variation in polar auxin transport (PAT) and elongating growth in etiolated Lupinus albus hypocotyls was investigated. Parameters of auxin transport, such as the amount transported, intensity of the transport and sensitivity to 1-N-naphthylphthalamic acid (NPA) inhibition were measured in isolated sections from different sites (apical, middle and basal) along the hypocotyls in seedlings of different ages. Auxin transport was studied by applying radioactive indole-3-acetic acid (IAA) to upright and inverted sections. Basipetal transport was much higher than acropetal and very sensitive to NPA inhibition, which indicates that transport is polarized. Polarity was expressed as the NPA-induced inhibition and the basipetal/acropetal ratio. As a rule, both the amount of IAA transported and the polarity varied with the age of the seedlings, with values increasing from 3 to 5d and then decreasing. Both parameters were higher in apical (where most growth is localized) than in middle and basal regions, although this longitudinal gradient tended to disappear with aging as hypocotyl growth slowed and finally ceased. The application of NPA did not modify hypocotyl elongation in 5-d-old intact seedlings. Derooting of the seedlings drastically reduced elongation in the control, while NPA partially restored the growth, which suggests that NPA induces an increase in auxin in the elongation region. These results suggest that a basipetally decreasing gradient in PAT along the hypocotyl, which changes with age, may be responsible for auxin distribution pattern controlling growth.     

Effect of Cyclanilide on Auxin Activity

Cyclanilide is a plant growth regulator that is registered for use in cotton at different stages of growth, to either suppress vegetative growth (in combination with mepiquat chloride) or accelerate senescence (enhance defoliation and boll opening, used in combination with ethephon). This research was conducted to study the mechanism of action of cyclanilide: its potential interaction with auxin (IAA) transport and signaling in plants. The activity of cyclanilide was compared with the activity of the auxin transport inhibitors NPA and TIBA. Movement of [³H]IAA was inhibited in etiolated corn coleoptiles by 10 μM cyclanilide, NPA, and TIBA, which demonstrated that cyclanilide affected polar auxin transport. Although NPA inhibited [³H]IAA efflux from cells in etiolated zucchini hypocotyls, cyclanilide had no effect. NPA did not inhibit the influx of IAA into cells in etiolated zucchini hypocotyls, whereas cyclanilide inhibited uptake 25 and 31% at 10 and 100 μM, respectively. Also, NPA inhibited the gravitropic response in tomato roots (85% at 1 μM) more than cyclanilide (30% at 1 μM). Although NPA inhibited tomato root growth (30% at 1 μM), cyclanilide stimulated root growth (165% of control at 5 μM). To further characterize cyclanilide action, plasma membrane fractions from etiolated zucchini hypocotyls were obtained and the binding of NPA, IAA, and cyclanilide studied. Cyclanilide inhibited the binding of [³H]NPA and [³H]IAA with an IC₅₀ of 50 μM for both. NPA did not affect the binding of IAA, nor did IAA affect the binding of NPA. Kinetic analysis indicated that cyclanilide is a noncompetitive inhibitor of both NPA and IAA binding, with inhibition constants (K _i) of 40 and 2.3 μM, respectively. These data demonstrated that cyclanilide interacts with auxin-regulated processes via a mechanism that is distinct from other auxin transport inhibitors. This research identifies a possible mechanism of action for cyclanilide when used as a plant growth regulator.     

Increased auxin efflux in the IAA-overproducing sur1 mutant of Arabidopsis thaliana: A mechanism of reducing auxin levels?

With the aim of investigating the mechanisms that maintain auxin homeostasis in plants, we have monitored the net uptake and metabolism of exogenously supplied indole-3-acetic acid (IAA) and naphthalene-1-acetic acid (NAA) in seedlings of wild type and the IAA-overproducing mutant sur1 of Arabidopsis thaliana . Tritiated IAA and NAA entered the seedling tissues within minutes and were mostly accumulated as metabolites, probably amino acid and sugar conjugates. The mutant seedlings were marked by a strong increase of [³H]IAA metabolism and a reduction of the accumulation levels of both free [³H]IAA and [³H]NAA. The same characteristics were observed in wild-type seedlings grown on 5 μ M picloram. We measured [³H]NAA uptake in the presence of high concentrations of unlabeled NAA or the auxin efflux carrier inhibitor naphthylphthalamic acid (NPA). This abolished the difference in free [³H]NAA accumulation between the mutant or picloram-treated seedlings and wild-type seedlings. These data indicated that active auxin efflux carriers were present in Arabidopsis seedling tissues. Picloram-treated seedlings and seedlings of the IAA-overproducing mutant sur1 displayed increased auxin efflux carrier activity as well as elevated conjugation of IAA. There is previous evidence to suggest that conjugation is a means to remove excess IAA in plant cells. Here, we discuss the possibility of efflux constituting an additional mechanism for regulating free IAA levels in the face of an excess auxin supply.     

NPA Inhibits Secretion of Amylases by Barley Aleurone Cells and Auxins can Overcome this Inhibition

Polar cell growth is regulated by auxin (Klämbt et al., 1992). It is essentially dependent upon exocytosis. Is exocytosis directly regulated by auxin? Aleurone cells secrete amylases, as well as other hydrolases, by exocytosis. In preliminary experiments, with 22 h of incubation, increasing concentration of N-1-naphthylphthalamic acid (NPA) parallels the inhibition of amylase secretion. A model system for secretion was designed. Barley aleurone layers, previously incubated in 1 μM GA₃ for 16 h, then washed extensively in the presence of 10 μM NPA for 10 min, were incubated finally for 2 h in GA₃, NPA and/or auxins. NPA inhibits amylase secretion. Auxins completely abolished secretion inhibition at the optimal concentration of 1 μM. Aleurone layers contain 1.5 to 3 μM endogenous indole-3-acetic acid (IAA) and have a high capacity to accumulate and retain IAA. A 7-fold accumulation occurred within 3 h after application of 20 μM radiolabeled IAA. A loss of about 10% could be measured within the following 4 h period in fresh media, independent of the presence of NPA. The inhibition of amylase secretion by NPA is not due to an inhibition of protein biosynthesis which was tested by the use of [³⁵S]-methionine.     

An Auxin Transport Inhibitor Interferes With Unicellular Gravitropism in Protonemata of the Moss Ceratodon purpureus

Abstract: Gravitropism of the protonemata of the moss Ceratodon purpureus (Hedw.) Brid. was studied after treatment with auxin transport inhibitors and auxin-related substances. The phytotropins NPA (naphthylphthalamic acid) and PBA (pyrenoylbenzoic acid), known to block auxin efflux in higher plants, strongly inhibited gravitropic curvature of the apical protonemal cell. At 3 μM NPA or PBA, approximately 60 % inhibition of curvature was observed; growth rates were less affected. Tyrphostin A47, a known antagonist of NPA effects in higher plants, released the inhibition of moss protonemal gravitropism and restored the full curvature response. Exogenous IAA, even at high concentrations (40 μM), did not interfere with protonemal gravitropism. To account for the results, modified hypotheses for auxin transport and action are discussed.     

Adaptation of corn roots to exogenously applied auxin

The ability of intact primary roots of corn ( Zea mays L. Bear Hybrid WF 9 × 38) to adapt to growth-inhibitory concentrations of auxin was studied using a highly sensitive position sensor transducer to measure growth. The timing, concentration dependence and temperature dependence of adaptation were studied as well as the time course of loss of adaptation upon removal of auxin. The rate of root elongation is inhibited 80% within 40 min after application of 10⁻⁷ M IAA. Within 90 min growth rate begins to recover. For concentrations of IAA equal to or greater than 10⁻⁷ M , recovery of growth rate (adaptation) is incomplete. Corn roots show a similar pattern of adaptation to the synthetic auxins NAA and 2,4-D. The Q₁₀ for adaptation is high (3.2) and comparable to that for root growth (3.3). Upon removal of exogenous IAA, loss of adaptation occurs with full sensitivity to the hormone regained within 20 min.
Based on the auxin specificity and the Q₁₀ for adaptation it is concluded that adaptation occurs neither by a change in the auxin degradation capacity of the root nor by a diffusional redistribution of applied auxin. It is suggested that adaptation involves metabolic processes, perhaps a metabolically dependent alteration of the number or affinity of auxin binding sites.     

Genetic Dissection of the Relative Roles of Auxin and Gibberellin in the Regulation of Stem Elongation in Intact Light-Grown Peas

Exogenous gibberellin (GA) and auxin (indoleacetic acid [IAA]) strongly stimulated stem elongation in dwarf GA1-deficient le mutants of light-grown pea (Pisum sativum L.): IAA elicited a sharp increase in growth rate after 20 min followed by a slow decline; the GA response had a longer lag (3 h) and growth increased gradually with time. These responses were additive. The effect of GA was mainly in internodes less than 25% expanded, whereas that of IAA was in the older, elongating internodes. IAA stimulated growth by cell extension; GA stimulated growth by an increase in cell length and cell number. Dwarf lkb GA-response-mutant plants elongated poorly in response to GA (accounted for by an increase in cell number) but were very responsive to IAA. GA produced a substantial elongation in lkb plants only in the presence of IAA. Because lkb plants contain low levels of IAA, growth suppression in dwarf lkb mutants seems to be due to a deficiency in endogenous auxin. GA may enhance the auxin induction of cell elongation but cannot promote elongation in the absence of auxin. The effect of GA may, in part, be mediated by auxin. Auxin and GA control separate processes that together contribute to stem elongation. A deficiency in either leads to a dwarfed phenotype.     

De novo shoot morphogenesis and plant growth of mustard (Brassica juncea) in vitro in relation to ethylene

Role of ethylene in de novo shoot morphogenesis from explants and plant growth of mustard ( Brassica juncea cv. India Mustard) in vitro was investigated, by culturing explants or plants in the presence of the ethylene inhibitors aminoethoxyvinylglycine (AVG) and AgNO₃. The presence of 20 μ M AgNO₃ or 5 μ M AVG in culture medium containing 5 μ M naphthaleneacetic acid and 10 μ M benzyladenine were equally effective in promoting shoot regeneration from leaf disc and petiole explants. However, AgNO₃ greatly enhanced ethylene production which reached a maximum after 14 days, whereas ethylene levels in the presence of AVG remained low during 3 weeks of culture. The promotive effect of AVG on shoot regeneration was overcome by exogenous application of 25 μ M 2-chloroethylphosphonic acid (CEPA), but AgNO_3-induced regeneration was less affected by CEPA. For whole plant culture, AVG did not affect plant growth, although it decreased ethylene production by 80% and both endogenous levels of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC by 70–80%. In contrast, AgNO₃ stimulated all 3 parameters of ethylene synthesis. Both AgNO₃ and CEPA were inhibitory to plant growth, with more severe inhibition occuring in AgNO₃. Leaf discs derived from plants grown with AVG or AgNO₃ were highly regenerative on shoot regeneration medium without ethylene inhibitor, but the presence of AgNO₃ in the medium was inhibitory to regeneration of those derived from plants grown with AgNO₃.     

Cooperation of Ethylene and Auxin in the Growth Regulation of Rice Coleoptile Segments

Elongation of coleoptile segments, having or not having a tip,excised from rice (Oryza sativa L. cv. Sasanishiki) seedlingswas promoted by exogenous ethylene above 0.3 µl l^–1as well as by IAA above 0.1 µM. Ethylene production ofdecapitated segments was stimulated by IAA above 1.0µM,and this was strongly inhibited by 1.0 µM AVG. AVG inhibitedthe IAA-stimulated elongation of the decapitated segment witha 4 h lag period, and this was completely recovered by ethyleneapplied at the concentration of 0.03 µl l^–1, whichhad no effect on elongation without exogenous IAA. The effectsof IAA and ethylene on elongation were additive. These factsshow that ethylene produced in response to IAA promotes ricecoleoptile elongation in concert with IAA, probably by prolongingthe possible duration of the IAA-stimulated elongation, butthat they act independently of each other. Moreover, AVG stronglyinhibited the endogenous growth of coleoptile segments withtips and this effect was nullified by the exogenous applicationof 0.03 µl l^–1 ethylene. These data imply that theelongation of intact rice coleoptiles may be regulated cooperativelyby endogenous ethylene and auxin in the same manner as foundin the IAA-stimulated elongation of the decapitated coleoptilesegments. Key words: oryza sativa, Ethylene, Auxin, Coleoptile growth     

Rapid-growth responses of corn root segments: Effect of auxin on elongation

The effect of indole-3-acetic acid (IAA) on the elongation rates of 2 mm corn (Zea mays L.) root segments induced by citrate-phosphate buffer (or unbuffered) solutions of pH 4.0 and 7.0 was studied. At pH 7.0, auxin initially reduced the elongation rate in both buffered and unbuffered solutions. Only in buffer at pH 7.0 was auxin at a concentration of 0.1 M found to promote the elongation rate though briefly. THis promoted rate represented only ca. 20% of the rate achieved with only buffer at pH 4.0. Auxin in pH 4.0 buffered and unbuffered solutions only served to reduce the elongation rates of root segments. Some comparative experiments were done using 2 mm corn coleoptile segments. Auxin (pH 6.8) promoted the elongation rate of coleoptile segments to a level equal or greater than the maximal H ion-induced rate. The two responses of root segments to auxin are compared to auxin action in coleoptile growth.     

Changes in the concentration of indole-3-acetic acid during the growth of etiolated lupin hypocotyls

The longitudinal distribution of unaltered radioactive indole-3-acetic acid (IAA), after application of [5-³H]-IAA to decapitated etiolated lupin hypocotyls. exhibited a wave-like pattern similar to that obtained with endogenous IAA. Waves of radioactive IAA were localizated both in the elongation zone and in the non-growing basal region of the hypocotyl. These IAA waves were transient because of basipetal polar transport and metabolism of IAA.
The level of endogenous IAA in different zones of the hypocotyl varied with age, following a wave-like pattern. During the elongation period of each zone, IAA was parallel to the bell-shaped curve of the growth rate. In addition, a role in secondary cell wall deposition is suggested for the other IAA wave that appeared after the cell elongation period, since an electron microscopic morphometric analysis of the cell wall showed that the cell wall thickness increased once the cell elongation ceased.
As the oscillation of endogenous IAA level occured in both space (distribution along the hypocotyl) and time (variation with age), it is suggested that the level of IAA really depended on the growth status of the cells. The response of the cells to the positional information submitted by the auxin waves as regards the growth status of the cell is discussed.     

A chlorate-hypersensitive, high nitrate/chlorate uptake mutant of Arabidopsis thaliana

N-ethylmaleimide (NEM) Lit 10-100 μ M led to a strong inhibition of the auxin-induced elongation growth of colcoptile segments, while fusicoccin-enhanced growth was not affected. Growth inhibition occurred only if NEM and auxin were allowed to act simultaneously. Preincubation of plant segments with NEM in the absence of auxin caused no inhibition of a subsequent growth stimulation by auxin, whenever NEM was removed before the application of IAA. However, preincubation with NEM plus auxin led to a remaining growth inhibition, which could not be reversed by a second auxin incubation in the absence of NEM. Fusicoccin added to NEM- plus auxin-treated segments was able to restore growth. It is suggested that auxin causes the unmasking of essential SH-groups of a protein to which NEM links covalently. thus inhibiting the growth process. This assumption was further supported by labeling experiments wish [¹⁴C]-NEM using membranes of maize ( Zea mays L. cv. Inraplus) coleoptiles. Two membrane fractions (S₂= 480-1900 g; S₄= 4300-15000 g) revealed a significantly higher [¹⁴C]-NEM labeling in the presence of auxin (2,4-diehlorophe-noxyacctic acid compared to 2,6 dichlorophenoxyacetic acid). This effect disappeared when the membranes were previously washed with EGTA [ethyleneglycolbis-(β-aminoethylether)-N,N,Nr',N'-tetraacetic acid]. The auxin-induced sensitization of coleoptilc segments against thiol-reagents and the auxin-induced expression of SH-groups of proteins of isolated membranes from coleoptiles arc suggested to be events involved in the primary action of auxins.     

The effect of auxin (indole-3-acetic acid) on the growth rate and tropism of the sporangiophore of <Emphasis Type="Italic">Phycomyces blakesleeanus</Emphasis> and identification of auxin-related genes

The roles of fungal auxins in the regulation of elongation growth, photo-, and gravitropism are completely unknown. We analyzed the effects of exogenous IAA (indole-3-acetic acid), various synthetic auxins including 1-NAA (1-naphthaleneacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid), and the auxin transport inhibitor NPA (N-1-naphtylphtalamic acid) on the growth rate and bending of the unicellular sporangiophore of the zygomycete fungus, Phycomyces blakesleeanus. Sporangiophores that were submerged in an aqueous buffer responded to IAA with a sustained enhancement of the growth rate, while 1-NAA, 2,4-D, and NPA elicited an inhibition. In contrast, sporangiophores kept in air responded to IAA with a 20 to 40% decrease of the growth rate, while 1-NAA and NPA elicited an enhancement. The unilateral and local application of IAA in the growing zone of the sporangiophore elicited in 30 min a moderate negative tropic bending in wild type C2 and mutant C148madC, which was, however, partially masked by a concomitant avoidance response caused by the aqueous buffer. Auxin transport-related genes ubiquitous in plants were found in a BLAST search of the Phycomyces genome. They included members of the AUX1 (auxin influx carrier protein 1), PILS (PIN-LIKES, auxin transport facilitator protein), and ABCB (plant ATP-binding cassette transporter B) families while members of the PIN family were absent. Our observations imply that IAA represents an intrinsic element of the sensory transduction of Phycomyces and that its mode of action must very likely differ in several respects from that operating in plants.     

The Elongation Response of Watermelon Hypocotyls to Indole-3-Acetic Acid: A Comparative Study of Excised Segments and Intact Plants

Carrington, C. M. S. and Esnard, J. 1988. The elongation responseof watermelon hypocotyls to indole-3-acetic acid: a comparativestudy of excised segments and intact plants.—J. exp. Bot39: 441–450. The auxin-growth response along the hypocotyl of Citrullus lanatus(Thumb.) Mansf. seedlings was studied. In excised segments,promotion of elongation was seen in all zones at the concentrationsof IAA used (10–4–10–2 mol m-3). In intactplants, only the most basal zone showed unequivocal IAA-extensionwhile in the most apical zone elongation was inhibited by auxin.This difference between segments and intact plants for apicalzones suggests a modifying effect of the apex and cotyledonson the growth response. Indeed, removal of the apex and colyledonsonly affected elongation in the zones adjacent to the excisionbut only in buffer-treated plants, not auxin-treated plants.Auxin supplied apically to the intact plant only resulted ina short-lived promotion of elongation whereas basally suppliedauxin gave a longer-lasting effect Zonal differences betweenauxin-promoted growth of excised segments suggests that sensitivityto auxin varies in the hypocotyl. The response of intact plantsto auxin was shown to be more complex than in segments. Thus,responses given by segments are poor indicators of auxin activityin intact plants. Key words: IAA, Citrullus lanatus, growth, plant hormone sensitivity     

Role of ethylene in auxin-induced flower bud formation in tobacco explants

The effect of ethytene on in vitro flower bud formation in thin-layer explants from tobacco pendicels ( Nicotiana tabacum L. cv. Samsun) was studied Endogenous ethylene production was stimulated by l-minocyclopropanc-l-carhoxylic acid (ACT), and inhibited by aminoethoxyviny lglycine (AVG). resulting in higher and lower ethylene accumulation. respectively. In the presence of an elevated ethylene concentration, the number of flower buds formed after 7 days of culture in explants was increased, compared with the control. Treatment with AVG or with AgNO₃ which blocks ethylene action resulted in decreased bud numbers after 7 days of culture. A different effect of ethylene was visible after 14 days of culture, when regeneration was complete. Treatment with AgNO₃ led to more bud regeneration, and increasing ethylene concentrations to lower bud numbers. The endogenous production of ethylene was enhanced by high concentrations of 1-naphthaleneacetic acid (NAA).
The inhibitory effect of applied ethylene was almost 100% in explants cultured at low concentrations of NAA (below 1 μ M ). but hardly visible at high concentrations (4.5 μ M ). As a consequence, the optimal NAA concentration shifted to a higher value in the presence of ethylene. These results are interpreted as a reduction in tissue sensitivity to auxin and in regenerative capability by ethylene. The effect of ethylene on auxin action is not exerted at the level of hormone concentration. Neither NAA uptake nor conversion to conjugates was effected by ethylene.     

Hormone growth responses of roots of shoot height mutants of pea

Growth of the roots vertical roots of 12 genotypes of peas ( Pisum sativum ), which differed in stem height because of differing gibberellin (GA) or brassinosteroid content or signal transduction, was recorded during continuous exogenous applications of differing concentrations of indoleacetic acid (IAA) and GA₃, over a period of 12 h with a sensitivity of 2 μm. IAA was inhibitory to all genotypes tested at concentrations of 0.5 n M and above. The response was rapid, reaching a maximum within 20 min. There was no response to 0.1 n M IAA, and no concentration tested promoted growth. At 0.5 n M the growth rate dropped to the same extent as with higher concentrations but then recovered slowly over about 12 h. Roots of nana ( na ) plants, which have reduced levels of IAA, partially recovered slowly from the inhibition of even 1 μ M IAA, to about 20% of the non-IAA growth rate after 10 h. When IAA (even 1 μ M ) was removed the growth rate of all lines slowly recovered over about 9 h after a lag of 1 h. IAA application to horizontal roots prevented bending in response to gravity. Roots of 3-day-old pea plants of all genotypes were unresponsive to exogenous GA but 5-day-old nana plants showed a gradual increase in elongation rate in response to GA, after a lag of a few hours, possibly because of a depletion of GAs provided from the cotyledons of the young plants. Roots in liquid culture did not branch and gradually became thinner. The growth of cultured roots was about 25% of that of intact plants of the same genotype. GA produced no effect on cultured roots, except in nana where it enhanced elongation. IAA was always inhibitory to growth in culture but stimulated branching. Some shoot-provided factor is necessary for the normal growth and development of the roots.